Frazzled commuters besieged radio talk-show programs yesterday to cheer Mayor Willie Brown's declaration of war on Critical Mass, the impromptu demonstration of cyclist solidarity that snarls San Francisco traffic on the last Friday of each month.

Fighting back in the escalating bicycle war, San Francisco's cyclists called Brown's move a betrayal of his earlier statements that he is committed to alternative transportation.

The fracas appears headed for a big showdown on July 25, when the next Critical Mass ride is scheduled.

Last Friday's Critical Mass event backed up traffic for more than a mile on the way to the Golden Gate Bridge. Since then, furious commuters have called in to KGO Radio, one of the Bay Area's more popular talk show stations, to vent their fury against the riders.

"We've been covering these events for several years, and people were as angry as I've ever heard them," said Bernie Ward, who hosts the station's early evening show. "It wasn't a question of cyclists sharing the road -- it was one of them shutting down every road they were on."

Brown said on Monday that he would crack down on the mass ride-along -- which can draw as many as 3,000 cyclists -- because it clogs the city's streets, stalling traffic and impeding pedestrians. He vowed to pull the police escorts now authorized for the event, and ticket cyclists who run red lights and commit other traffic infractions.

Ward said Bay Area residents are solidly behind the mayor's move against Critical Mass because it appears riders are receiving inexplicably preferential treatment.

"If anybody else shut down one block in this city for a fair, you could wallpaper your house with the permits they'd have to get," said Ward. "But it's completely hands-off for Critical Mass. People resent it."

Riders, on the other hand, say they feel that Brown has slipped a rapier in their backs.

"We didn't expect it," said Raf, a cyclist who was taking a break at Harvey's Place on 5th Street, a favorite bike messenger haunt. "We thought he was on our side, but this is a fascist stunt. He can take a busload of mayors out there and screw up traffic as much as he wants, but he dumps on us for riding our bikes."

Cyclist Fred Geitz also had harsh words for the mayor. "He showed he is on the side of motor vehicles," Geitz said. "I was in New York years ago when they tried to restrict bike messengers and other cyclists. It didn't work there, and it won't work here."

Many in the city, however, are siding with Brown.

"Critical Mass has become an issue for businesses all over the city, not just in the downtown area," said Carol Piasente, a spokeswoman for the San Francisco Chamber of Commerce. "We've heard from our members that business is affected and that employees are having a hard time getting to and from work."

Piasente said her organization supports alternative transportation, but that it has to be regulated.

"Brown's position that cyclists have to follow the rules of the road seems reasonable to us," she said.

At a City Hall press conference yesterday, Brown said he is getting nothing but positive feedback from the public, and that he hasn't heard from any Critical Mass representatives. He reiterated that cyclists must follow same rules as everybody else, challenging cyclist claims that they are simply doing what cars do all the time:

"If you're in a car and you run a red light or go down a one-way street the wrong way, you get busted," Brown said. "Keep it up, and you're accused of reckless driving, which means going to jail or getting your license suspended."

Erik Beckjord, an urban planning consultant and bike enthusiast, has suffered insults and threats from his fellow riders for supporting the crackdown on Critical Mass.

"They have this attitude that anyone in a car is a capitalist pig, and anyone who rides a bike is pure and good and can disregard the laws," Beckjord said. "That's ridiculous. Brown has done the right thing (by cracking down on Critical Mass), but he needs to follow through. We need more cops and more enforcement."

Beckjord will not answer his phone calls now without monitoring them on an answering machine. He played a recent message from a cyclist outraged by his anti- Critical Mass stance. "Now I have your phone number, and I know where your business is," said the anonymous caller. "You'll be hearing from us very soon, pal."

"I'm definitely concerned about violence," Beckjord said.

Adam Gubser, the assistant bicycle program manager for the San Francisco Department of Parking and Traffic, said the city's mean streets make easy solutions to the bicycle/car conflict impossible. "'An incredibly built-up area like San Francisco makes it very tough to come up with answers that will please everyone," he said.

Some leaders of San Francisco's cycling community worry that Brown's throwing down the gauntlet will only make matters worse. "If you think there's gridlock now, wait until 2,000 cyclists start stopping at every red light," said Joe Carroll, the events coordinator for the San Francisco Bicycle Coalition. "If Brown wants to do something about congestion, he should fund more bike lanes and bike racks."